Surface inspection system with improved capabilities

ABSTRACT

Pixel intensities indicative of scattered radiation from portions of the inspected surface surrounding a location of a potential anomaly are also stored so that such data is available for quick review of the pixel intensities within a patch on the surface containing the location of the potential anomaly. Where rotational motion is caused between the illumination beam and the inspected surface, signal-to-noise ratio may be improved by comparing the pixel intensities of pixels at corresponding positions on two different surfaces that are inspected, where corresponding pixels at the same relative locations on the two different surfaces are illuminated and scattered radiation therefrom collected and detected under the same optical conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/615,918, filed Oct. 4, 2004, which application is incorporated byreference as if fully set forth herein.

BACKGROUND OF THE INVENTION

This invention relates in general to defect detection, and, inparticular, to an improved system for detecting anomalies on surfaces,such as particles and surface-originated defects such ascrystal-originated particles (“COPs”) and other defects.

The Sp1^(TBI)™ detection system available from KLA-Tencor Corporation ofSan Jose, Calif., the Assignee of the present application, isparticularly useful for detecting defects on unpatterned semiconductorwafers. The SP1^(TBI) system provides the capability to review areas ofthe surface where potential anomalies have been identified, in a modeknown as microview. The SP1^(TBI) system determines the presence ofpotential anomalies by comparing intensities of detector outputs with athreshold. If the intensity of radiation detected by the detectorexceeds the threshold, a potential anomaly is then determined to bepresent at the location from which the scattered radiation is detected.The intensity value is then stored and identified to be indicative of apotential anomaly at a corresponding location of the surface.

After the entire surface (such as the surface of a semiconductor wafer,reticle or display panel) has been inspected in this manner, the areascontaining locations where the potential anomalies are found are thenre-examined to determine whether anomalies are actually present at suchlocations. In this determination, it is useful and often necessary tocompare the radiation scattered by the surface at the locationsdetermined to have potential anomalies to radiation scattered byportions of the surface surrounding such locations, such as portionswithin patches or areas of the surface, each patch or area containingone of such locations. In the microview mode, the SP1^(TBI) systemre-scans the portions of the surface surrounding the locationsdetermined to have potential anomalies, and records the detector outputintensities so obtained. This is necessary because the scatteredradiation intensities detected by the detectors from areas surroundingthe potential anomalies (and where no potential anomaly has beendetected) fall below the threshold and are therefore discarded and notstored. A comparison of the detector output intensities of radiationscattered by the surface at the locations determined to have potentialanomalies to the detector output intensities of radiation scattered byportions of the surface surrounding such locations will confirm whetheranomalies are indeed present at such locations. These confirmedlocations with potential anomalies can then be examined in greaterdetail or at higher resolution. The microview mode is useful since theremay be a large number of locations with potential anomalies, and there-scanning and comparison process in this mode may reduce the number oflocations that would need to be examined in greater detail or at higherresolution.

While the above-described microview mode of the SP1^(TBI) system isuseful, it requires re-scanning of the surface. The SP1^(TBI) systemprovides unsurpassed defect sensitivity on bare wafers or unpatternedwafers; however, this is not the case when it is used for inspectingwafers with patterns thereon such as wafers with memory arrays, or forinspecting surfaces with much back ground noise.

Where rotational motion is caused between the illumination beam and thesurface that is being inspected, it may be difficult to perform what isknown as die-to-die comparison between pixel intensities of twodifferent areas on the same surface that is being inspected. This is dueto the fact that the angle of illumination of the two areas may bedifferent, since the areas may be illuminated at different azimuthalangles, and the collection angles (both azimuthal and elevation) ofscattered radiation may also differ between the two areas, depending onthe timing of the rotation. If the two areas contain pattern, thepattern then may be at different orientations relative to theillumination beam and the collection optics so that a subtraction of thepixel intensities of the two areas or patches does not normally reducenoise caused by scattering due to pattern.

It is therefore desirable to provide a surface inspection system withcapabilities that are better than those outlined above.

SUMMARY OF THE INVENTION

This invention is based on the recognition that the above-describeddifficulties encountered in the microview mode of the SP1^(TBI) systemcan be overcome by storing, in addition to information concerningradiation scattered from locations of the surface determined to havepotential anomalies, also information concerning radiation scatteredfrom portions of the surface adjacent to such locations and notdetermined to have potential anomalies. This is in contrast to thecurrent microview mode of the SP1^(TBI) system where only theintensities of the radiation scattered from locations of the surfacedetermined to have potential anomalies are stored. In this manner,subsequent to the scan, if the user desires to view radiation scatteredfrom areas adjacent to the location of the potential anomaly, this ispossible without having to re-scan the surface.

Another aspect of the invention is based on the recognition that whiledie-to-die comparison of areas on the same surface may be difficult whenrotational motion is caused between the illumination beam and thesurface inspected during inspection, it may still be possible to comparecorresponding areas of two different surfaces that are inspected so thatthe performance of the surface inspection system can be much improvedfor inspecting surfaces with similar patterns thereon. The two surfacesmay be scanned sequentially or simultaneously by a beam or beams ofradiation. Radiation scattered from at least a portion of the firstsurface is collected and radiation scattered from at least a portion ofthe second surface is also collected. Preferably the two portions of thetwo surfaces have substantially the same relative locations on the firstand second surfaces and substantially the same relative orientation withrespect to the beam or beams and the collection optics collectingradiation scattered from the two respective areas. The radiationcollected from the portions of the two surfaces or signals derivedtherefrom are used to determine or confirm the presence of potentialanomalies in or on the portions of the first and/or the second surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the SP1^(TBI) system, where rotationaland translational motion is caused between a radiation beam and theinspected surface, useful for illustrating the invention.

FIG. 2 is a schematic diagram illustrating a convergent hollow cone ofradiation useful for illustrating the invention.

FIG. 3A is a schematic view of a possible arrangement of multiple fiberchannels for carrying scattered radiation collected by the ellipsoidalcollector of the system of FIG. 1 useful for illustrating the invention.

FIG. 3B is a schematic view of an multi-anode photomultiplier tube (PMT)that can be used in conjunction with an arrangement of multiple fiberchannels such as that shown in FIG. 3A useful for illustrating theinvention.

FIG. 4 is a schematic view of an arrangement of fiber channels/multipledetectors for carrying scattered radiation collected by the lenscollector in the narrow channel of the system of FIG. 1 useful forillustrating the invention.

FIG. 5A is a cross-sectional view of a defect inspection system usefulfor illustrating the invention.

FIG. 5B is a cross-sectional view of an arrangement of separate opticalchannels used in the embodiment of FIG. 5A.

FIG. 6A is a cross-sectional view of an alternative defect inspectionsystem useful for illustrating the invention.

FIG. 6B is a cross-sectional view of an arrangement of segmented opticalchannels used in the embodiment of FIG. 6A.

FIG. 7 shows partially in perspective and partially in block diagramform a system for inspecting anomalies of a semiconductor wafer surface,where two dimensional translational motion is caused between a radiationbeam and the inspected surface, useful for illustrating the invention.

FIG. 8 is a perspective view showing in more detail the illumination andcollection features of the system of FIG. 7.

FIG. 9A is a schematic view of a surface that is inspected by a surfaceinspection system used for illustrating different aspects of theinvention.

FIG. 9B is a schematic view of a surface that is inspected a surfaceinspection systems which, together with FIG. 9A, are useful forillustrating an aspect of the invention where the collected radiationfrom both surfaces in the two figures are employed for anomalydetection.

FIG. 10A is a schematic view of a defect map of a semiconductor waferwith logic circuits thereon to illustrate the invention.

FIG. 10B is an intensity map of the semiconductor wafer of FIG. 10A,where radiation scattered from areas surrounding the defects are shownin addition to the defects themselves to illustrate the invention.

FIGS. 10C and 10D are views of patches on the semiconductor wafer ofFIGS. 10A and 10.

FIG. 11 is a collection of 32 different views of radiation in differentdirections from the same area of a surface that is inspected having apattern thereon.

FIG. 12 is a view of radiation detected from a surface with a patternand 0.8 micron polystyrene latex spheres thereon.

FIGS. 13A-13D are graphical plots useful for illustrating the improvedperformance when signals from two surfaces having the same patternthereon are compared for defect detection on both surfaces.

For simplicity in description, identical components are labeled by thesame numerals in this application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The description below in reference to FIGS. 1 through 6B is similar tothe description of similar figures in U.S. Pat. No. 6,538,730, which isincorporated herein in its entirety by reference. The optical systems ofthe patent are useful for illustrating the invention.

FIG. 1 is a schematic view of the SP1^(TBI) system 10 available fromKLA-Tencor Corporation of San Jose, Calif., the assignee of the presentapplication. Aspects of the SP1^(TBI) system 10 are described in U.S.Pat. Nos. 6,271,916 and 6,201,601, both of which are incorporated hereinin their entireties by reference. To simplify the figure, some of theoptical components of the system have been omitted, such as componentsdirecting the illumination beams to the wafer. The wafer 20 inspected isilluminated by a normal incidence beam 22 and/or an oblique incidencebeam 24. Wafer 20 is supported on a chuck 26 which is rotated by meansof a motor 28 and translated in a direction by gear 30 so that beams 22and/or 24 illuminates an area or spot 20 a which is caused to move andtrace a spiral path on the surface of wafer 20 to inspect the surface ofthe wafer. Motor 28 and gear 30 are controlled by controller 32 in amanner known to those skilled in the art. Alternatively, the beam(s) 22,24 may be caused to move in a manner known to those skilled in the artto trace the spiral path or another type of scan path.

The area or spot 20 a illuminated by either one or both beams on wafer20 scatters radiation from the beam(s). The radiation scattered by area20 a along directions close to a line 36 perpendicular to the surface ofthe wafer and passing through the area 20 a is collected and focused bylens collector 38 and directed to a photomultiplier tube (“PMT”) 40.Since lens 38 collects the scattered radiation along directions close tothe normal direction, such collection channel is referred to herein asthe narrow channel and PMT 40 as the dark field narrow PMT. Whendesired, one or more polarizers 42 may be placed in the path of thecollected radiation in the narrow channel.

Radiation scattered by spot 20 a of wafer 20, illuminated by either oneor both beams 22, 24, along directions away from the normal direction 36is collected by an ellipsoidal collector 52 and focused through anaperture 54 and optional polarizers 56 to dark field PMT 60. Since theellipsoidal collector 52 collects scattered radiation along directionsat wider angles from the normal direction 36 than lens 38, suchcollection channel is referred to as the wide channel. The outputs ofdetectors 40, 60 are supplied to a computer 62 for processing thesignals and determining the presence of anomalies and theircharacteristics. In the conventional operation of the SP1^(TBI) system,the intensities of the outputs of detectors 40, 60 are compared tothreshold(s). When such detector output intensities exceed thethreshold(s), the locations from which scattered radiation is detectedby the detectors to provide such outputs then potentially haveanomalies, and both the locations and the corresponding intensities ofthe detector outputs are stored.

Microview Without Re-Scan

As noted above, in the conventional operation of the SP1^(TBI) system,both the detector output intensities as well as the locations from whichscattered radiation is detected by the detector(s) to provide suchoutput intensities are not stored unless such locations have beendetermined to contain potential anomalies, typically by comparing suchintensities to a certain threshold. When surfaces with pattern or noisybackground are inspected, there may be a large number of locationsdetermined to have potential anomalies. In order to further examine thelocations determined by conventional operation of the SP1^(TBI) systemto potentially have anomalies, it may be necessary to review all suchlocations at high resolution. Such operation, however, is time consumingso that it may be desirable to briefly review each of the locations in amicroview mode before they are examined at high resolution. Such reviewnormally requires knowledge of the intensities of the scatteredradiation from areas surrounding the locations identified to havepotential anomalies. During the conventional operation of the SP1^(TBI)system, such information is not recorded so that during a conventionalmicroview mode, the user would have to rescan such locations and thesurrounding areas in order to record the scattered radiation from thesurrounding areas as well as scattered radiation from the locations.This is cumbersome and time consuming.

One aspect of the invention is based on the recognition that, instead ofrecording only the detector output intensities that exceed certainthresholds, one would record and store the detector output intensitiesfrom portions of the surface in the vicinity of locations with potentialanomalies where such intensities do not exceed the preset thresholds. Inthis manner, data indicating the scattered radiation intensities fromportions of the wafer surface surrounding locations containing potentialanomalies is available for review immediately after the inspection ofthe surface. The user can then quickly examine an area or patch of thewafer surface containing locations of potential anomalies for confirmingwhether such locations contain potential anomalies (and therefore meritsdetailed and/or high resolution examination) without having to re-scanthe wafer surface. Thus, in reference to FIG. 1, in one embodiment,computer 62 stores in its associated memory (not separately shown) thedetector output intensities provided by the detector or detectors inresponse to scattered radiation from all portions of the wafer, and notjust the detector output intensities from locations where potentialanomalies were identified.

To economize on the amount of memory that may be required for storage,it may be desirable to erase or discard the detector output intensitiesfurther away from locations containing potential anomalies. This isillustrated in reference to FIG. 9A.

FIG. 9A is a schematic view of a semiconductor wafer surface where apotential anomaly has been determined to exist at location 302. Topermit further review of the detector output intensity data at location302 as well as the detector output in response to scattered radiationfrom areas surrounding location 302 for confirmation of the potentialanomaly at the location, it may be adequate to store the detectoroutputs corresponding to radiation scattered from an designated area 304containing location 302. Then the detector output intensities inresponse to scattered radiation from portions of the wafer surface 300outside the area 304 may be discarded or erased if no location in thevicinity of such portions is identified to contain a potential anomaly.Thus if the surface 300 contains more than one location with potentialanomalies, areas or patches similar to patch 304 may be designated whereeach of the areas or patches contains one of the locations and storageof the detector output intensities will be retained only for portions ofsurface 300 within such areas or patches so that the remaining data maybe erased. For example, where the shape of the patch is circular (e.g.patch 306 in FIG. 9A) with radius d, and the detector output intensitiesare recorded in the form of pixels, the pixel data may be erased if nopotential anomaly has been identified on the surface within distance dfrom such pixel. The area designated or distance d may be set prior tothe inspection.

The capability of the surface inspection system to perform microviewwithout re-scanning the surface is not limited to systems such as theSP1^(TBI), where both locational and translational motion is causedbetween the illumination beam or beams and the surface inspected. Thusall of the above features of recording detector output intensities foreither the entire wafer or those corresponding to patches may beapplicable to systems where two dimensional translational motion iscaused between the illumination beam or beams and the surface inspectedas described in more detail below.

The above-described microview without requiring re-scan capability canbe further enhanced where scattered radiation from the surface that isbeing inspected is collected and detected at a different number ofdirections at different azimuthal and/or elevation angles, in a schemeknown as acquiring a multi-perspective view of the surface as describedbelow.

The microview without re-scan feature is illustrated in FIGS. 10A-10D.FIG. 10A is a schematic view of a defect map of a semiconductor waferwith logic circuits thereon provided in a conventional microview moderequiring re-scan. A defect map is constructed by identifying the pixelintensities of the detector outputs with the pixel locations of thesurface from which scattered radiation is detected to provide suchintensities, and such map can be displayed as shown in FIG. 10A. Asdescribed above, the detector intensity outputs are stored in theconventional microview mode only when such intensities fall above apredetermined threshold. Thus, to obtain a defect map, computer 62compares detector intensities on the surface to a predeterminedthreshold or thresholds and identify the locations on the surface 20where intensity of scattered radiation exceeds the threshold orthresholds. Such locations together with their associated detected pixelintensities are then reported to form the defect map shown in FIG. 10A.The potential defects are shown as dots on the map, such as dot 302.

FIG. 10B illustrates a map of the semiconductor wafer whose defect mapis shown in FIG. 10A. FIG. 10B is obtained using the improved techniquedescribed above where re-scan of the wafer is unnecessary. FIG. 10Billustrates detector the output intensities of scattered radiationdetected from the entire wafer of FIG. 10A, where the detector outputintensities are shown not only for locations with potential anomalies,but intensities for all locations on the wafer.

For the purpose of recording and storing the detector output intensitiesfor the construction of maps such as those shown in FIGS. 10A and 10B,the surface of the wafer is divided into pixels and an output intensityvalue is associated with each pixel. The maps of FIGS. 10A and 10B arethen constructed using such pixel intensity values. The location 302 ofa potential anomaly of FIG. 9A is also shown in FIGS. 10A and 10B. FIG.10C is a view of an area or patch of the defect map of FIG. 10A wherethe patch or area contains the location 302. Unlike the defect map ofFIG. 10A, however, the patch view in FIG. 10C shows the defect 302 as awhite dot on a dark background.

According to an embodiment of one aspect of this invention, when theuser wishes to also observe the pixel intensities in an area of patch ofthe wafer map containing the defect 302, all the user would have to dois to double click the computer mouse (not shown) at location 302 on acomputer screen displaying the map in FIG. 10A, and computer 62 wouldpresent a view showing all the pixel intensities within the patch orarea of the wafer in FIG. 10A, as shown in FIG. 10D. As noted above,since computer 62 has stored in its memory either all the pixelintensities of the entire surface 20, or at least the pixel intensitiesin areas or patches containing the locations of potential anomalies, allof the pixel intensities shown in FIG. 10D would be stored in the memoryof computer 62, so that the re-scanning of the surface 20 would beunnecessary in the improved microview mode. Thus as can be seen fromFIG. 10D, viewing the potential defect at location 302 in the context ofits surroundings makes it easier for the user to determine whether ananomaly indeed exists at location 302. If the user determines that apotential anomaly indeed exists at location 302, then this portion ofthe surface can be further examined at higher resolution or with evenmore elaborate measures to examine this location and its surroundingsfor determination and classification of the anomaly. By avoiding thenecessity to re-scan the wafer, however, this process of identifying andreviewing potential anomalies is much faster and less cumbersome thanconventional microview.

Rotational Symmetry

The SP1^(TBI) system is advantageous for unpatterned wafer inspectionsince the collection optics (lens 38 and mirror 52) is rotationallysymmetric about the normal direction 36, so that the orientation of thesystem in FIG. 1 relative to the orientation of defects on the surfaceof wafer 20 is immaterial. In addition, the angular coverage of thescattering space by these collectors is well matched to those requiredto detect the anomalies of interest in unpatterned wafer inspectionapplications.

In addition to the above characteristic, however, the SP1^(TBI) system10 has another important characteristic in that both its lens collector38 and the ellipsoidal mirror collector 52 preserve the azimuthalinformation contained in radiation scattered by defects on surface ofwafer 20. Thus, certain defects and/or pattern on the wafer may scatterradiation preferentially along certain azimuthal directions more thanother azimuthal directions. By making use of the preserved azimuthalinformation in the collected radiation by the collectors 38 and 52,system 10 may be advantageously adapted and modified for the detectionof defects on patterned wafers.

By segmenting the radiation collected by the lens 38 and/or ellipsoidalmirror 52, radiation scattered in different azimuthal directions may bedetected separately. In this manner, the detectors detecting radiationdiffracted or scattered by pattern may become saturated, while otherdetectors not detecting such diffraction or scatter will yield usefulsignals for the detection and classification of defects on wafer 20.Since the lens 38 and ellipsoidal mirror 52 preserve the azimuthalinformation of the scattered radiation, knowledge of the type of patternor defects present on wafer 20 can be advantageously used to design andposition multiple detectors to advantageously detect and classify thedefects on the wafer. This is especially true in the case of regularpatterns such as memory structures on wafer 20, as will be explainedbelow, since radiation diffracted by such regular patterns also tend tobe regular.

FIG. 2 is a schematic view illustrating a convergent hollow cone ofradiation which can be collected by lens 38 or mirror 52. In the case oflens 38 of FIG. 1, a spatial filter (not shown in FIG. 1) is employed toblock the specular reflection of the normal incidence beam 22 fromreaching detector 40, so that the radiation focused by lens 38 to PMT 40has the shape of a convergent hollow cone illustrated in FIG. 2. In thecase of the ellipsoidal mirror 52, since the mirror is not a completeellipse, it collects only radiation scattered at larger angles to thenormal direction 36 without also collecting the radiation scattered atnear normal directions, so that the radiation focused by mirror 52towards detector 60 also has the shape of a convergent hollow cone asshown in FIG. 2.

FIG. 3A is a schematic view of a possible arrangement of multiple fiberchannels receiving radiation in the convergent cone of radiation shownin FIG. 2, such as that collected by mirror 52, to illustrate thepreferred embodiment of the invention. The arrangement in FIG. 3Acomprises two substantially concentric rings of optical fiber channels72 that are used to carry the collected scattered radiation in theconvergent hollow cone shown in FIG. 2. Fourier components or otherpattern scattering from the pattern on the wafer 20 may reach some ofthe fibers 72, thereby causing the detectors detecting the radiationfrom such channels to be saturated or provide large amplitude signals.However, there will be other optical fiber channels that do not receivesuch unwanted pattern scattering. The use of multiple fiber channels 72effectively segments the collected scattered radiation into differentsectors or segments so that only some of the fiber channels will receivea strong signal and can become saturated or provide high amplitudeoutputs due to the Fourier or other pattern scatter leaving theremaining channels carrying information that can be analyzed fordetecting anomalies. As will be explained below, since the azimuthalinformation in the collected scattered radiation in the cone of FIG. 2is preserved, various schemes may be employed to minimize the effects ofthe pattern scatter when the segmented approach of FIG. 3A is used.

Different types of detectors may be used to detect the radiation carriedby the fiber channels 72, such as the multi-anode PMT shown in FIG. 3B.In the event a multi-anode PMT is used, however, there is a nominalthree percent cross-talk between any two adjacent channels. To avoidsuch cross-talk, fibers 72 may be aligned with every other PMT anode, ina manner illustrated in FIG. 3B. FIG. 3B is a schematic view of amulti-anode PMT. As shown in FIG. 3B, only the anodes 74 that are shadedare aligned with fibers 72, where anodes 76 are not aligned with any ofthe fibers 72. This avoids the three percent cross-talk that may bepresent if all of the anodes shown in FIG. 3B are aligned with fibers72.

FIG. 4 is a schematic view illustrating an arrangement 80 of fiberchannels or multiple detectors 82 for the narrow channel. Thus, fibersor detectors 82 may be aligned with the collected scattered radiationillustrated in FIG. 2 for the narrow channel collected by lens 38 forsegmenting the radiation in a similar manner as that described above forthe wide channel.

FIG. 5A is a partially cross-sectional view and partially schematic viewof a defect inspection system to illustrate the preferred embodiment ofthe invention. To simplify FIG. 5A, the two illumination beams 22 and24, computer 62 and the mechanisms for moving the wafer are not shown inthe figure. Radiation scattered by spot 20 a on wafer 20 and collectedby lens 38 is reflected by mirror 102 to detector 40. Stop 104 blocksthe specular reflection of the normal incident beam 22 from detector 40and results in a cone shape of the convergent beam in FIG. 2. The beamcollected and focused by lens 38 and reflected by mirror 102 passesthrough a beam splitter 106 and a portion of the collected radiationthat passes through the beamsplitter is focused onto detector 40 toprovide a single output as would be the case in normal SP1^(TBI)operation. Beamsplitter 106 reflects and diverts a portion of thecollected radiation from lens 38 to the arrangement 80 of optical fibersof FIG. 4. Preferably, the size of optical fibers 82 and the size of thehollow cone reflected by beamsplitter 106 are such that fibers 82collect and convey most of the radiation in the hollow cone ofradiation. Each of the fibers 82 is then connected to a correspondingdetector or a detecting unit in a multi-unit or multi-element detector.In a similar manner, beamsplitter 112 diverts a small portion of theradiation collected by ellipsoidal mirror 52 towards arrangement 70′ ofoptical fiber channels 72, shown more clearly in FIG. 5B (or FIG. 3A),where each channel 72 is connected to a separate detector or a separatedetecting unit in a multi-element detector system (not shown). As shownin FIG. 5A, beamsplitter 112 is such that it diverts radiation onlywithin a narrow ring 114 to arrangement 70′. Most of the radiationcollected by mirror 52 is passed through beamsplitter 112 and focused todetector 60 to provide a single output as would be the case in normalSP1^(TBI) operation. In FIG. 5A, the illumination beams 22, 24 and themechanisms for moving the wafer have been omitted to simplify thefigure.

As will be evident from a comparison of system 10 of FIG. 1 and system100 of FIG. 5A, system 100 retains substantially all of the features ofsystem 10 of FIG. 1. In addition, system 100 diverts a portion of thescattered radiation collected by each of lens 38 and mirror 52, anddirects it towards fibers 82, 72 to convey the segmented radiation toseparate detectors or detecting units. The system is compact andrequires minimal additional space compared to the SP1^(TBI) system 10 ofFIG. 1. In this manner, a single combined instrument may be optimizedand used for both unpatterned and patterned wafer inspection, therebyeliminating the need for two separate instruments for the two types ofwafer inspection.

When only patterned wafers are to be inspected, an alternative defectinspection system 150 of FIG. 6A may be used. In FIG. 6A, theillumination beams 22, 24, computer 62 and the mechanisms for moving thewafer have been omitted to simplify the figure. As shown in FIG. 6A,scattered radiation collected by lens 38 and by mirror 52 are reflectedby mirror 112′ towards an arrangement of optical fibers 152 which isshown more clearly in cross-section in FIG. 6B. As shown in FIG. 6B,arrangement 152 includes a ring of fibers 82 conveying scatteredradiation collected by lens 38 and a ring of fibers 72 conveyingscattered radiation collected by mirror 52. As before, each of thefibers 72, 82 may be connected to a separate detector or a detectingunit of a multi-unit detector.

While a single ring of detectors are shown in FIGS. 4 and 5B, multiplerings may be employed such as that shown in FIG. 3A. The opticallytransmissive cores of optical fibers that are located adjacent to eachother in each of the two arrangements 70, 70′, 80 are separated fromeach other by the claddings that envelope the cores so that crosstalkbetween adjacent cores is reduced. Obviously, optical channels otherthan fibers may be used and are within the scope of this invention.Where such channels do not include separators such as the cladding inthe case of optical fibers, other optical separators may be employed toreduce crosstalk.

Systems 100 and 150 of FIGS. 5A and 6A are particularly advantageous fordistinguishing between micro-scratches and particles. The scatteringpattern due to a micro-scratch gives the highest concentration of energyand greatest detection uniformity when illuminated normally and capturedin the near normal or narrow channel collected by lens 38. The uniquesignature of the scratch in the form of an elongated pattern in thefar-field, allows for a simple method of classification. Therefore, ifthe eight or more fibers 82 arranged in a ring format is placed in thepath of the hollow cone of light focused by lens 38 towards fibers 82 asdiverted by beamsplitter 106, where the outputs of these fibers aredirected onto a multi-channel detector or an array of individualdetectors, by a simple process of comparing the signals obtained throughany two diagonally opposed fibers relative to the signals in theremaining fibers, the presence of the micro-scratch is obtained. Whenilluminated obliquely, micro-scratches result in scattering patternswhich can be distinguished from those due to particles, by using themultiple detection channels that were described above in conjunctionwith pattern inspection, viz. multiple fiber units 70 and 70′. In boththe wide and narrow channels, it is also possible to place individualdetectors or multi-element detecting systems directly in the path of theconverging hollow cone of light, rather than individual optical fibers.

In the manner described above, the collection space for scatteredradiation from the surface may be segmented in the azimuthal directions.In a similar manner, the collection space for scattered radiation fromthe surface may be segmented in the elevation directions as well, whichare defined by the elevation angles of such directions relative to thesurface inspected. For example, the elevation collection angles offibers 82 (e.g. 82 a in FIG. 5A) are different from those of fibers 72(e.g. 72 a in FIG. 5A), and elevation collection angles of the innerring of fibers 72 in FIG. 3A are different from those of fibers 72 inthe outer ring.

Multiperspective Applied to Surfaces with Pattern

Where systems 100, 150 are used for inspecting wafers with memory cellsthereon, the Fourier components in the radiation scattered by the memoryarray will spin as the wafer is rotated. These components will thusrotate and be at different azimuthal angles about the normal direction36 of FIGS. 1, 5A and 6A. This means that these Fourier components willbe conveyed by different fibers 72, 82 as the wafer is rotated. Sincethe array of memory cells may have different dimensions in the X and Ydirections of the wafer, as the wafer rotates, the number of detectorsthat are saturated by the Fourier components may change. This can beprovided for by knowing the X and Y dimensions of the memory cells sothat the number of Fourier diffraction components can be estimated.Alternatively, during an initialization process at the beginning, alearn cycle is performed where the maximum number of Fourier componentsthat need to be eliminated is determined by noting the maximum number ofdetectors with very strong, or saturated, outputs. During the subsequentmeasurement after initialization, this number of detector outputs maythen be eliminated, where the outputs eliminated are the ones that aresaturated or the ones that have the largest values. In the case of amulti-anode PMT, for example, where each anode is used and is connectedto a corresponding fiber, cross-talk may be reduced by also eliminatingthe components adjacent to the detectors having the highest outputs. Forexample, if the wafer in one position gives three Fourier components,and in another two, the three direct components together with twocomponents adjacent to each would be eliminated for a total of ninedetector outputs that are eliminated. This leaves seven useable detectoroutputs. This number will be maintained regardless of the exactorientation of the wafer. This allows the user to maintain the sizingoption for the particles.

Preferably the fibers 72 and 82 are arranged rotationally symmetricallyaround a direction, such as axes 74 and 84 shown in FIGS. 3A, 4, 5B and6B. When arranged in such manner, the radiation scattering directionsare partitioned into identical angular segments and radiation scatteredwithin each segment is collected by a corresponding fiber. Whenbeamsplitter or mirror 102, 112, 112′ reflects or diverts a portion ofthe radiation collected by lens 38 or mirror 52, the azimuthal positionsof the collected scattered radiation is preserved when the reflected ordiverted radiation is directed to the fibers 72, 82. When such radiationis so reflected or diverted, axes 74, 84 correspond to the normaldirection 36, and the azimuthal positions of the collected scatteredradiation about the axes 74, 84 corresponding to their azimuthalpositions about the normal direction 36 are preserved. It is alsopossible to collect scattered radiation at different elevation angles(i.e. at different angles from the surface inspected). For example,fibers 72 collect radiation scattered at elevation angles different fromthose at which scattered radiation is collected by fibers 82. Thisprovides multiperspective views of the scattered radiation originatingfrom the illumination beam(s) from the inspected surface.

Microview and Multiperspective

The above described features providing multiperspective views (i.e.portions of maps) of the scattered radiation from the inspected surfacecan be combined with the above described microview capability. Forexample, any one of the systems 10, 100 and 150 of FIGS. 1, 5A and 6Amay be used to scan the surface 20, and the intensities of the detectoroutputs are stored (such as in memory associated with computer 62),whether or not such intensities exceed predetermined thresholds. Uponcompletion of the scan, it is possible for a user to obtain a microviewof an area or patch containing the location of interest (such aslocation 302 in FIG. 10A) at different elevation and/or azimuthalangles. This is illustrated, for example, in FIG. 11. FIG. 11 is acollection of 32 views (microviews) of the same patch or area of theinspected surface obtained by detecting intensities of radiationsscattered by the patch into 32 different directions. Thus, the 32directions may differ from one another by azimuthal angle, elevationangle from the inspected surface, or both. As can be observed from FIG.11, the pixel intensity values in some of the views of the patches (312,314, 316 and 318) contain high intensity scattered radiation. This mayindicate that the views 312-318 contained scattered radiation frompattern or other periodic irregularities of surface 20. For this reason,these four views may be ignored in the determination of the existence ofanomalies on the surface 20. As noted above, different types of defectsor anomalies will scatter radiation at different preferential elevationor azimuthal angles. By providing a large number of views from differentcollection angles (both azimuthal and elevation), much more informationis available for determining both the presence and the type of anomaliesthat may be present on surface 20.

In reference to FIG. 11, where the surface inspected contains pattern orother noisy background, the various collection channels at differentcollection angles will show different perspectives of the same patch orarea on the inspected surface. Patterns such as memory arrays on thesurface cause strong scattering in particular preferential directions.To reduce the affect of pattern scattering on anomaly detection, it maybe desirable to discard detector outputs that are saturated or where thepixel intensity is too high to be caused by scattering from anomalies.Thus one may discard all together the views 312-318. Of the remainingviews, it may be desirable to provide a suitable value for each pixel inthe area or patch, such as by computing a weighted average of the pixelintensities from the different multiperspective views of the same areaor patch. In one embodiment, this value may also be a median value.Alternatively, the minimum value among the remaining views may beselected instead. Thus a view of an area or patch of the inspectedsurface may be formed by the weighted average, median or the minimumpixel intensities of all the views that remain after discarding the onesthat are saturated or having pixel intensities that are too high.

While it is possible for the systems described above to store andprovide a large number of views of the entire surface inspected inmulti-perspective, the amount of memory required can be substantial andtoo costly for certain applications. This may be the case even wherepixel intensities are stored only for patches or areas containinglocations of potential anomalies while discarding or erasing pixelintensities that are further away from such locations. Thus for someapplications, it may be desirable to first scan the surface using only asingle or a small number of collection channels and correspondingdetectors, such as system 10 of FIG. 1. After a defect map such as thatshown in FIG. 10A has been compiled using the results of the scan, thepatches or areas containing locations of potential defects may bere-scanned with a larger number of collection channels and correspondingdetectors than the number used during the previous scan, such as in thesystems illustrated in FIGS. 2-6B, so that a larger number of differentperspective views of these patches at different collection angles can beobtained. This process may be advantageous for certain applications.

Microview Using Different Inspection System

Another type of surface inspection system will now be described inreference to FIGS. 7 and 8. The description of this type of system isset forth in more detail in U.S. Pat. Nos. 6,215,551 and 5,864,394,which are herein incorporated by reference in their entireties. As shownin FIG. 7, system 220 includes a laser 222 providing a laser beam 224.Beam 224 is expanded by beam expander 226 and the expanded beam 228 isdeflected by acousto-optic deflector (AOD) 230 into a defected beam 232.The deflected beam 232 is passed through post-AOD and polarizationselection optics 234 and the resulting beam is focused by telecentricscan lens 236 onto a spot 210 on surface 240 to be inspected, such asthat of a semiconductor wafer, photomask or ceramic tile, patterned orunpatterned.

In order to move the illuminated area 210 on surface 240 for scanningthe entire surface, the AOD 230 causes the deflected beam 232 to changein direction, thereby causing the illuminated spot 210 on surface 240 tobe scanned along a scan line 250. As shown in FIG. 7, scan line 250 ispreferably a straight line having a length which is smaller than thedimension of surface 240 along the same direction as the scan line. Evenwhere line 250 is curved, its span is less than the dimension of surface240 along the same general direction. After the illuminated spot hascompleted scanning surface 240 along scan line 250, surface 240 of thewafer is moved by means of stage 244 (see FIG. 8) along the X axis sothat the illuminated area of the surface moves along arrow 252 and AOD230 causes the illuminated spot to scan along a scan line parallel toscan line 250 and in adjacent position spaced apart from scan line 250along the negative X axis. After the illuminated spot has covered suchscan line, surface 240 is moved by a small distance by means of stage244 so that the area of the surface to be illuminated is moved alongdirection 252 in order to scan an adjacent scan line at a different Xposition. This small distance preferably is equal to about one quarterof the height of spot 210. This process is repeated until theilluminated spot has covered strip 254; at this point in time theilluminated area is at or close to the edge 254 a. At such point, thesurface 240 is moved along the Y direction by about the length of scanline 250 by means of stage 244 in order to scan and cover an adjacentstrip 256, beginning at a position at or close to edge 256 a. Thesurface in strip 256 is then covered by short scan lines such as 250 ina similar manner until the other end or edge 256 b of strip 256 isreached at which point surface 240 is again moved along the Y directionfor scanning strip 258. This process is repeated prior to the scanningof strip 254, 256, 258 and continues after the scanning of such stripsuntil the entire surface 240 is scanned. Surface 240 is thereforescanned by scanning a plurality of arrays of short path segments thetotality of which would cover substantially the entire surface 240.

The deflection of beam 232 by AOD 230 is controlled by chirp generator280 which generates a chirp signal. The chirp signal is amplified byamplifier 282 and applied to the transducer portion of AOD 230 forgenerating sound waves to cause deflection of beam 232 in a manner knownto those skilled in the art. For a detailed description of the operationof the AOD, see “Acoustooptic Scanners and Modulators,” by MiltonGottlieb in Optical Scanning, ed. by Gerald F. Marshall, Dekker 1991,pp. 615-685. Briefly, the sound waves generated by the transducerportion of AOD 230 modulates the optical refractive index of anacoustooptic crystal in a periodic fashion thereby leading to deflectionof beam 232. Chirp generator 280 generates appropriate signals so thatafter being focused by lens 236, the deflection of beam 232 causes thefocused beam to scan along a scan line such as line 250 in the mannerdescribed.

Chirp generator 280 is controlled by timing electronic circuit 284 whichin the preferred embodiment includes a microprocessor. Themicroprocessor supplies the beginning and end frequencies f1, f2 to thechirp generator 280 for generating appropriate chirp signals to causethe deflection of beam 232 within a predetermined range of deflectionangles determined by the frequencies f1, f2. The auto-position sensor(APS) optics 290 and APS electronics 292 are used to detect the level orheight of surface 240. Detectors such as detector 211 b collects lightscattered by anomalies as well as the surface and other structuresthereon along scan line 250 and provides output signals to a processorin order to detect and analyze the characteristics of the anomalies.

FIG. 8 is a perspective view of system 220 of FIG. 7 showing in moredetail the arrangement of the collection or detection channels toillustrate the preferred embodiment. As shown in FIG. 8, four collectionchannels and corresponding detectors are used, two channels—detectors210 a, 210 b (detectors not shown separately from the collectionchannels in FIG. 8) for collecting scattered light that is within therespective ranges of azimuthal angles of −(75-105) degree and (75-105)degree. Two additional collection channels-detectors 211 a, 211 b arealso employed for detecting forward scattered light that is within therespective ranges of azimuthal angles of −(30-60) degree and (30-60)degree. If desired, it is of course possible to employ four independentcollection channels with other different solid angles of collection, twoof said collection channels located in the forward direction to collectlight in the forward direction centered substantially at ±45 degreeazimuthally and two of the channels are located to collect lightcentered substantially at ±90 degree azimuthally. More or fewercollection channels and detectors than four can be employed as well.These channels may collect at different azimuthal and/or elevationangles.

The above-described process of achieving microview without re-scan (withor without multiperspective) can also be performed using the system ofFIGS. 7 and 8. In the same manner as that described above of themicroview mode without re-scan using the systems of FIGS. 1-6B,processor 200 may store in its memory the pixel intensities indicativeof the scattered radiation from areas or patches of the inspectedsurface containing a location of a potential anomaly even though suchpixel intensities do not exceed a preset threshold to indicate thepresence of anomalies. In this manner, after the scan, all of the pixelintensities within the areas or patches may be viewed without having tore-scan the inspected surface 240. If desired, of course, the pixelintensities of the entire surface that is inspected may be stored. Alsoin the same manner as that described above, since multiple collectionchannels and corresponding detectors are available in the systems ofFIGS. 7 and 8, it is also possible to acquire multi-perspective views ofareas or patches of the surface 240 that is being inspected. It istherefore possible to store the pixel intensities for all the differentperspectives so that the microview capability may be performed for eachone of the perspectives or for only selected ones.

Surface-to Surface (e.g. Wafer-to-Wafer) Comparison

In reference to FIGS. 9A and 9B, the two surfaces 300 and 300′ containsubstantially the same pattern thereon. Where the two surfaces aresurfaces of semiconductor wafers, such surfaces typically have alignmentmarks such as flats 320 and 320′. Thus the pattern on surface 300 islocated relative to flat 320 at the same relative orientation andlocation as the orientation and location of a similar pattern on surface300′ relative to the flat 320′. Instead of using flats as alignmentmarks, notches (not shown) may be used instead. Thus when surface 300 isinspected by means of the optical systems in FIGS. 1-6B as well as thosein FIGS. 7 and 8, the inspection system has stored in its computer suchas computer 62 the relative orientations and locations of theillumination beam or beams and the collection channels relative toalignment mark 320. When surface 300′ is inspected, in like manner, suchsystem would also record the relative orientations and locations of theillumination beam or beams and the collection channels relative to thealignment mark 320′. In this manner, if surfaces 300 and 300′ areinspected sequentially by the same inspection system, it is possible tobe relatively certain that the area or patch 306′ on surface 300′ isilluminated in a manner (e.g. orientation relative to the illuminationbeam(s)) which is substantially the same as the illumination of patch orarea 306 of surface 300. It is also possible to be relatively certainthat the radiation scattered from area or patch 306′ would be detectedalong collection directions that are substantially the same as those forcollecting radiation scattered from the area or patch 306 on surface 300in the prior scan. Areas or patches 306 and 306′ are thereforecorresponding patches of the two surfaces.

It is then possible to reduce noise and improve signal-to-noise ratio bytaking advantage of pixel intensities for both areas or patches 306 and306′. In one embodiment, the intensities of the pixels in area 306 maybe compared to the intensities of the pixels at the same relativelocations in area 306′ where the comparison may be a simple subtraction.This may be performed for any two corresponding areas or patches of thetwo surfaces, and may indeed be performed for all of the pixels of theentire two surfaces 300 and 300′. While the above-described process isviable where surfaces 300 and 300′ are inspected sequentially by thesame inspection system, the same or similar advantages may also beobtained where the two surfaces are inspected by two differentinspection systems, if the two surfaces are inspected in such a mannerthat the two corresponding areas or patches 306 and 306′ compared aresubject to the same illumination and collection conditions by the twodifferent inspection systems. Then the pixel intensities atcorresponding pixels in the two areas 306 and 306′ may be compared orotherwise used to improve the signal-to-noise ratio in the same manner.

Defects of the above-described comparison are illustrated in FIGS. 12and 13A-13D. FIG. 12 is an intensity pixel map obtained by scanning asurface with pattern and polystyrene latex (PSL) spheres of 0.8 micronsdiameter thereon. The white dots on plots in FIG. 12 indicate thepresence of such spheres.

FIG. 13A is a graphical plot of the surface whose pixel map is shown inFIG. 12. In FIG. 13A, two of the peaks 330 and 332 of curve 340 havehigh pixel intensities indicative of the strength of radiation scatteredby the PSL spheres. FIG. 13B is a graphical plot of the pixelintensities obtained when the same surface scanned to provide the plotin FIG. 13A is re-scanned after the PSL spheres have been removed, wherethe pixel intensities after removal is shown as curve 342. Superimposedas a thinner (compared to curve 342) line onto FIG. 13B is the curve 340from FIG. 13A. As will be apparent from the map in FIG. 12 and acomparison of the plots in FIGS. 13A and 13B, after the PSL spheres areremoved, some of the peak intensities (e.g. 330 and 332) in curve 340 donot occur in curve 342. However, curve 342 still contains high amplitudeintensity pixel values, such as at 342 a, which appear to overlap peak340 a of curve 340. This means that the surface that is being inspectedto yield the results shown contain sources of strong scattering otherthan the PSL spheres, such as a pattern. For this reason, even after thePSL spheres are removed, curve 342 still contains a number of high pixelintensities.

FIG. 13C is a graphical plot of the two curves shown in FIG. 13B butafter pixel intensities have been processed by interpolation andsmoothing where the resulting curves are indicated as 340′ and 342′. Aswill be apparent from FIG. 13C, mere interpolation and smoothingalgorithms are inadequate in reducing noise caused by strong scatteringfrom pattern. FIG. 13D is a graphical plot of the pixel intensities thatremain after the two curves 340 and 342 are shifted to overlap moreexactly with one another and after the intensity values of one curve aresubtracted from those of the other at the same locations of the surface.Since both curves will contain pixel values due to scattering ordiffraction from the same pattern or other surface irregularities, thesubtraction of pixel intensities of corresponding pixels in the twoscans results in a much cleaner curve, where the pixel intensities dueto the scattering from pattern or other surface irregularities are muchreduced. As a result, the signal caused by the scattering from the PSLspheres becomes much more prominent and noticeable. In other words, thesignal-to-noise ratio is much improved for the detection of particledefects on the surface by a comparison of the two curves 340 and 342(e.g. by subtraction). The results illustrated in FIGS. 12-13D thereforeillustrate the above described feature of the invention. The areas orpatches 306 and 306′ contain similar pattern or other surfaceirregularities, so that radiation scattered by such irregularities orpattern give rise to similar pixel intensities at the same correspondingpixel locations. By subtracting the pixel intensities of correspondingpixels in the two areas or patches, the signal-to-noise ration will besimilarly increased. Obviously, comparisons other than by simplesubtraction may also be used; such and other variations are within thescope of the invention.

While the invention has been described above by reference to variousembodiments, it will be understood that changes and modifications may bemade without departing from the scope of the invention, which is to bedefined only by the appended claims and their equivalent. All referencesreferred to herein are incorporated by reference in their entireties.

1. A surface inspection method for detecting anomalies on a surface,comprising: causing relative motion between a beam of radiation and thesurface; collecting radiation scattered from the surface and directingthe collected radiation to one or more channels; converting thecollected radiation carried by at least one of the channels intorespective signals; determining presence of potential anomalies in or onthe surface from said signals; storing information in the signalsconverted from radiation scattered from locations of the surfacedetermined to have potential anomalies and information in the signalsconverted from radiation scattered from portions of the surface adjacentto such locations and not determined to have potential anomalies;designating dimensions of an area prior to the collecting andconverting, wherein the storing stores at least information in signalsconverted from radiation scattered from different pixels that are on thesurface and that are within one or more areas of the designateddimensions, each of said areas containing a location on the surfacewhere a potential anomaly is determined to be present; and erasingstored information in signals converted from radiation scattered fromportions on the surface only when such portions are not within any oneof the areas each containing a location on the surface where a potentialanomaly is determined to be present.
 2. The method of claim 1, whereinthe storing stores an image of at least a portion of the surface thathas not been determined to be potentially anomalous.
 3. The method ofclaim 1, wherein the determining identifies presence of potentialanomalies in or on the surface by comparing intensities of said signalsto at least one threshold and said storing stores information in thesignals collected from at least a portion of the surface whereintensities of said signals are not more than said at least onethreshold.
 4. The method of claim 1, wherein the causing causes relativetranslational motion between a beam of radiation and the surface alongtwo directions transverse to each other so that the beam scanssubstantially the entire surface.
 5. The method of claim 1, furthercomprising retrieving the stored information and providing a map ofpixels in a portion of the surface where one or more potential anomalieshave been determined to be present, said pixels in the portioncomprising ones at locations where presence of potential anomalies havenot been determined to be present.
 6. The method of claim 5, wherein themap is provided without re-scanning the surface.
 7. The method of claim1, wherein said collecting collects radiation scattered from the surfaceat different elevation angles from the surface or at different azimuthalangles about a line normal to the surface or about a directioncorresponding thereto, so that information related to relative elevationpositions from the surface or at azimuthal positions of the collectedradiation about the line or is preserved, wherein said directing directsradiation scattered by the surface to a plurality of different channelsso that at least some of the channels carry radiation scattered atdifferent elevation angles or at different azimuthal angles with respectto the line, wherein the converting converts the collected radiationcarried by said at least some of the channels into respective signals;and wherein said determining determines presence of potential anomaliesin or on the surface from said respective signals.
 8. The method ofclaim 7, further comprising retrieving the stored information andproviding a plurality of maps of pixels in a portion of the surfacewhere one or more potential anomalies have been determined to bepresent, each of the maps containing information related to thecollected radiation scattered from the portion of the surface at acorresponding azimuthal angle about the line and/or elevation angle fromthe surface.
 9. The method of claim 7, wherein the surface has a patternthereon and wherein the determining determines by means of detectorsthat provide output signals in response to the collected radiationcarried in the corresponding channels, and determines presence ofpotential anomalies without using output signals of detectors above apredetermined threshold.
 10. The method of claim 7, wherein thedetermining determines by means of detectors that provide output signalsin response to the collected radiation carried in the correspondingchannels, and the determining determines presence of potential anomaliesin or on the surface from a weighted average, median or minimum value ofat least some of the detector output signals provided in response to thecollected radiation carried in the channels from the same location ofthe surface.
 11. The method of claim 10, wherein the storing storesinformation related to the weighted average, median or minimum values ofthe detector output signals provided in response to the collectedradiation carried in the channels scattered from locations of thesurface determined to have potential anomalies, and information relatedto the weighted average, median or minimum values of the detector outputsignals provided in response to the collected radiation carried in thechannels scattered from portions of the surface that are determined notto have potential anomalies and that are adjacent to such locations. 12.The method of claim 1, wherein said collecting collects radiationscattered from the surface at different azimuthal angles about a linenormal to the surface or about a direction corresponding thereto whenthe surface is scanned by the beam so that information related torelative azimuthal positions of the collected radiation about the lineis preserved, wherein said directing directs radiation scattered by thesurface to a plurality of different channels so that at least some ofthe channels carry radiation scattered at different azimuthal angleswith respect to the line, wherein the converting converts the collectedradiation carried by said at least some of the channels into respectivesignals; and wherein said determining determines presence of potentialanomalies in or on the surface from said respective signals.
 13. Themethod of claim 12, wherein the storing stores information from therespective signals so that stored information from one signal convertedfrom collected radiation scattered from a portion of the surface at afirst corresponding azimuthal angle about the line is distinguishablefrom stored information from another signal converted from collectedradiation scattered from the same portion of the surface at a secondcorresponding azimuthal angle about the line different from the firstcorresponding azimuthal angle.
 14. The method of claim 12, wherein thecollecting and directing the collected radiation to channels is by meansof a collector that collects the scattered radiation substantiallysymmetrically about the line or the direction.
 15. The method of claim1, wherein said directing comprises focusing the collected radiation tooptical fibers that serve as the channels by means of at least oneobjective.
 16. The method of claim 15, wherein said directing comprisesreflecting a portion of the collected radiation at different azimuthalangles from a reflective collector towards the channels.
 17. A surfaceinspection method for detecting anomalies on a surface, comprising:causing relative motion between a beam of radiation and the surface;collecting radiation scattered from the surface and directing thecollected radiation to one or more channels; converting the collectedradiation carried by at least one of the channels into respectivesignals; determining presence of potential anomalies in or on thesurface from said signals; and storing information in the signalsconverted from radiation scattered from locations of the surfacedetermined to have potential anomalies and information in the signalsconverted from radiation scattered from portions of the surface adjacentto such locations and not determined to have potential anomalies;setting a distance, wherein the storing stores at least information insignals converted from radiation scattered from different pixels thatare on the surface and that are within such distance from any locationon the surface where a potential anomaly is determined to be present;and erasing the stored information in signals converted from radiationscattered from a pixel on the surface only when no potential anomalieshave been determined to be present within said distance from such pixel.18. A surface inspection method for detecting anomalies on a surface,comprising: causing relative motion between a beam of radiation and thesurface; collecting radiation scattered from the surface and directingthe collected radiation to one or more channels; converting thecollected radiation carried by at least one of the channels intorespective signals; determining presence of potential anomalies in or onthe surface from said signals; storing information in the signalsconverted from radiation scattered from locations of the surfacedetermined to have potential anomalies and information in the signalsconverted from radiation scattered from portions of the surface adjacentto such locations and not determined to have potential anomalies;re-scanning portions of the surface at and adjacent to potentialanomalies determined to be present by a beam of radiation; collectingradiation scattered from said portions of the surface during the re-scanand directing the collected radiation to each channel of a group of thechannels, where the number of channels in the group is higher than thenumber of channels previously used for determining the presence ofpotential anomalies; and confirming or discounting presence of potentialanomalies in or on the surface from signals converted from radiationcarried in the group of channels.